python error in addon "Load Obj Sequence as Shape Keys"
Well, the first thing to notice is that

target.data.shape_keys.key_blocks


is a null or empty object at the point of the error and thus the script cannot execute the for loop to fill the sknames object.

Upon looking into this further it seems the issue is with the file path that references the location of the obj files around line 56 (at least it did for me).

Replacing this:

for f in files:
    fp = spath[0] + "\\" + f


with this:

for f in files:
    fp = os.path.join(spath[0], f)


seemed to solve it for me.